Waveguide grating routers are used in conventional lightwave systems as optical switches, multiplexers, demultiplexers, detectors, add/drop filters, one by N (1.times.N), N by one (N.times.1) splitters and N by N (N.times.N) arrays. Typically, such waveguide grating routers include an interconnection apparatus having a plurality of closely spaced input waveguides communicating with an input of a star coupler. An output of the star coupler communicates with an optical grating comprising a series of optical waveguides. Each of the grating waveguides differs in length with respect to its nearest neighbor by a predetermined fixed amount. The optical grating is further connected to an input of a second star coupler, the outputs of which form outputs of the switching, multiplexing, and demultiplexing apparatus. Waveguide grating routers are also frequently referred to as "frequency routing devices" and are further described in U.S. Pat. No. 5,002,350, issued Mar. 26, 1991 to C. Dragone, entitled "Optical Multiplexer/Demultiplexer" (hereinafter "Dragone 1") and U.S. Pat. No. 5, 136,671, issued Aug. 4, 1992 to C. Dragone, entitled "Optical Switch, Multiplexer, and Demultiplexer" (hereinafter "Dragone 2"), both of which are hereby incorporated by reference.
Such frequency routing devices are employed as wavelength routers in optical networks to drop and add optical channels at various nodes in an optical network. A particular channel may pass through several routers without regeneration before leaving the network. For this reason, it is desirable for each frequency routing device to provide a maximally flat passband.
Known frequency routing devices do not efficiently provide a flat passband between a distinct one of the input ports and a distinct one of the output ports. U.S. Pat. No. 5,412,744, issued May 2, 1995 to C. Dragone, entitled "Frequency Routing Device Having Wide and Substantially Flat Passband" (hereinafter "Dragone 3"), and hereby incorporated by reference, discloses a frequency routing device in which a flat passband is achieved by combining a frequency routing device with a Y-branch coupler. One limitation of this device, however, is that there is an inherent loss of optical power due to the presence of the coupler.
Thus, there is a need for a waveguide grating router having a substantially flat passband, without the disadvantages of prior art systems.